The present invention relates to a solar cell module comprising a plurality of solar cells and constituted by connecting adjacent solar cells to one another by wiring line members (leads), and a method of manufacturing the module.
In recent years, there have been increasing expectations for clean energy in view of protection of global environments from global warming or the like. A solar cell module which converts solar energy directly into electric energy is noted as a clean energy source. In this case, the solar cell module comprises a plurality of solar cells, and, for example, wiring line members referred to as tab leads are connected to these adjacent solar cells with a solder (or paste) to electrically connect the cells to one another.
In this case, there are used a series of wiring line members ranging from the surface of one of the adjacent solar cells to the back surface side of the other solar cell (see, e.g., Japanese Patent Application Laid-Open No. 2004-363293 (Document 1)). There is also a case where a wiring line member connected to the surface of one solar cell is connected to a wiring line member connected to the back surface side of the other solar cell (see, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2002-359388 (Document 2)).
This wiring line member is constituted of a copper foil usually having a width of 1.5 mm to 2 mm and a thickness of about 150 μm (microns) to 200 μm, and the member is prepared in a state in which the member is wound around a bobbin in manufacturing steps. The wiring line member is coated beforehand with a solder, the member having a predetermined dimension is drawn from the bobbin and cut, and the member laid on the solar cell is heated with hot air, a lamp heater or the like, and soldered.
Here, by the subsequent natural cooling, the wiring line member expanded by heat when soldered, and the solar cell of the corresponding portion contract. On the other hand, a linear expansion coefficient of copper is about 16.7×10−6/° C., whereas a silicon substrate of the solar cell has a linear expansion coefficient of about 2.33×10−6/° C. Therefore, the wiring line member after the soldering contracts more largely than the solar cell, and an acting force accompanying a difference in this contraction is applied to the solar cell.
On the other hand, since the wiring line member is wound around the bobbin as described above, the drawn wiring line member has a curled shape. Moreover, the member meanders in some case. This also applies to a bent wiring line member. Such nonlinear wiring line member in which the curled or meandered state is generated deviates from a predetermined soldered position on the solar cell and a collector electrode. There is a possibility that characteristics deteriorate owing to an appearance defect or reduction of an irradiated area. Therefore, heretofore the cut wiring line member is pulled with a predetermined load, and straightened out, or the wiring line member is cut after the straightening out.
After the wiring line member is soldered to the solar cell, the acting force accompanying the above-described contraction difference is applied to the solar cell, but the forces acting between the wiring line member on the front surface side of the solar cell and the solar cell and between the wiring line member on the back surface side and the solar cell act in a direction in which the forces cancel each other. However, it is supposed that the forces are not equal to each other and, as a result, the solar cell is warped.
In addition, a conventional single-crystal or polycrystal solar cell has a thickness of about 300 μm to 350 μm. Therefore, even when the acting force due to the contraction of such wiring line member is applied, the solar cell module is hardly warped as shown in FIG. 5. It is to be noted that in this drawing, reference numeral 1 denotes solar cells, and 2 denotes wiring line members.
However, a thickness of a wafer for general use in the solar cell is about 150 μm to 200 μm, and is remarkably small for the purpose of reducing material costs. Therefore, there occurs a problem that the contraction of the wiring line members 2 acts, and the solar cell module largely warps as shown in FIG. 6.
For example, in a solar cell (the solar cell preferably has a size of 125 mm square or more) constituted of a crystalline wafer having an about 125 mm square size and a thickness of about 200 μm, it is seen that warpage reaches as much as 3 mm which is defined by the longest distance D between a horizontal line L of FIG. 6 and the solar cell 1. When such warpage is generated, an operation efficiency deteriorates in the manufacturing steps. Moreover, damages are generated such as breaks or cracks. Therefore, a manufacturing yield drops and the decrease in reliability is feared.
To solve this problem, a material such as titanium whose linear expansion coefficient is close to that of the solar cell is used as the wiring line member in Document 1. However, such metal is remarkably expensive as compared with copper, and an electric resistance value of the metal is higher than that of copper. Therefore, the metal needs to be combined with another substance, and there is a further problem that the member costs rise.